I can race around town all day no problem and the temp stays below half.. I can drive on the freeway for several hours and the temp stays below half.. but as soon as i get OFF the freeway after driving for about 30-45 minutes and start to slow down or come to a stop, it gets hot.. so I turn on the heater for a few minutes and keep driving and its fine. Coolant is full, replaced thermostat, clutch fan is good, and coolant is circulating.. I know my catalytic converter isn't the best it could be, could this be the problem? Otherwise, how do I go about flushing the system? Any recommendations would be appreciated.